What is a firewall and what are the main types?

A firewall is a network security device that monitors and controls incoming and outgoing traffic based on predetermined rules. Packet filtering firewalls inspect individual packets against access control lists based on source and destination IP addresses, ports, and protocols. Stateful inspection firewalls track the state of active connections and make decisions based on context. Application layer firewalls, also called proxies, inspect the content of traffic at layer seven and can filter based on application-specific data.